I ran into roughly the same thing. I sent in a PR on 13-June as
a placeholder. It's kern/10358
I'm pretty sure it depends upon which chip/board you are using.
From the PR:
The DE500-BA has a 21143. I can't get it to work in full duplex
on either Alpha or i386. (Half duplex works as expected.)
The DE500-AA has a 21140A and DP83840. It works correctly, both
full and half. (Only tested on Alpha.)
The motherboards on Miatas and XP1000s also have 21143s. They
work correctly both full and half.